1. Field of the Invention
The invention relates to an oscillation damper with a variable damping force including a valve device and an actuator for the valve device which performs a rotary motion against the spring force of a torsion spring.
2. Description of the Related Art
Oscillation dampers with a variable damping force achieve a large adjustment range as regards the damping force by having a plurality of valve devices or because an adjusting device can itself cover a large adjustment range. A large adjustment range can be achieved by implementing a significantly large difference in damping force with just a small adjusting movement of the valve device. The difference in damping force generally corresponds to a difference in the cross section of a valve opening. However, relatively high accuracy in the adjustment of the valve device is required for this purpose. If only a small difference in damping force is achieved in relation to an adjusting movement, it is possible either to apportion the damping force more finely or to use components that are less accurate.
However, a large adjustment travel is also associated with a more powerful adjusting device, which is correspondingly expensive. U.S. Pat. No. 6,293,377 describes an oscillation damper with an adjustable damping force, in which an electric motor is connected by its rotor to a driving-force conversion mechanism that converts a rotary motion of the rotor into an axial motion of a valve element. It is completely irrelevant for the action of the force of an electric motor how large the adjustment travel is. An adjusting device of this kind therefore appears particularly advantageous. In this connection, attention is drawn to DE 34 08 267, which has already described a driving-force conversion mechanism. When using an electric motor, the angle of rotation of the rotor required to achieve a defined damping-force setting must be determined. One known method of achieving this is to use a stepping motor. The special way in which this type of motor can be activated results in a defined angle of adjustment of the rotor, which, in turn, corresponds to a defined adjustment travel of the valve element. However, to the knowledge of the applicant, stepping motors of this kind are not available in the range of diameters that would be necessary for implementation within a hollow piston rod.
Another possibility is to have the electric motor run into a stop in order to define an angle of rotation. However, it is only possible to define a small number of angles of rotation in this way. A solution of this kind is known from DE 35 42 846 A1, for example.
The use of stops to determine the position of a valve element in conjunction with an electric motor involves another problem, which can incidentally also arise if the valve element is embodied as a seat valve that can be moved into the maximum closed position. When the stop is encountered or the valve seat is occupied, very high impact torques occur, these going far beyond the rated motor torque. Either a certain wear is accepted or use is made of a control system of the type known from GB 2,187,346, which describes an electrical circuit that significantly reduces the motor torque before the stop is reached and hence brakes the stop.